An annular gap scrubber or washer generally comprises a housing or duct forming an elongated passage of tapering cross section, e.g. with an inlet of small cross section, an outlet of larger cross section and a frustoconical section of increasing cross section between the inlet and the outlet, a frustoconical body being mounted in this passage to define an annular gap with the frustoconical section.
This body is axially displaceable so that the width of the annular gap can be adjusted and, upstream of the gap and this body, one or more spray nozzles can be disposed for spraying the scrubbing liquid, usually water, into the dust-entraining gas stream.
The frustoconical portion of the passage is termed a diffusor because, as this term is used in nozzle applications, it diverges in the direction of flow of the gas stream. The inlet passage, outlet passage and diffusor can form the housing of the annular gap washer although they can be constituted by separate ducts opening into another housing.
In general, the diffusor and the body adjustably disposed therein are of circular cross section and various formations can be provided at the leading end of this body for guidance or other purposes.
While annular gap washers are described in the aforementioned patents and application and the literature cited or of record in the files thereof, reference may be had to German utility model No. 76 27 885 which illustrates a typical annular gap scrubber. Characteristic of substantially all of these scrubbers is the fact that over substantially the entire length of the gap, the gap width is constant, i.e. the inner surface of the housing surrounding the central body and defining the gap therewith is geometrically similar to the outer surface of the body.
In some cases, however, it has been suggested that this gap width should increase in the direction of flow of the gas, i.e. that the surfaces defining the gap should diverge from one another in the direction of flow.
The fluid flow through the gap thus is not accelerated although acceleration may occur as the fluid mixture enters the gap.
Where the surface of the body and the juxtaposed surface of the housing wall are geometrically similar, these surfaces may be said to have identical base angles.
The term "base angle" is used here to describe the angle included between the surface and a plane perpendicular to the axis of the scrubber at the largest end of the surface. If, for example, the body is a frustocone, the base angle is the angle between the outer surface of this body and the large base of the frustocone. A corresponding base angle can be assigned to the housing surface. The angle is thus defined between a generatrix of the conical surface and a radius at the point along the surface where this radius is largest within the region over which the gap extends.
This arrangement in which the acceleration of the gas is reduced to zero or less within the gap has been found to be especially advantageous when the annular gap washer is not only a device for the cleaning of a gas stream but simultaneously serves as the control element of a control system for the gas stream, e.g. for the pressure at the head of a pressurized blast furnace as described in the aforementioned patents.
However, when the central body of the annular gap washer is mounted so as to be displaceable in the manner described, mechanical vibrations or oscillations of this body develop which can interfere with the proper function of the scrubber both as a gas washer and as a pressure control element, and can damage the support system for the central body and other parts thereof.
Depending upon how this body is mounted, these vibrations can be quite considerable.